In recent years, from the viewpoint of global environment problems, there have been many attempts to suppress use of fossil fuels, and as one of them, a solar heat-collecting system which utilizes solar heat is known. As such a solar heat-collecting system, for example, a solar heat-collecting system may be mentioned which comprises a heat collection tube provided with a heat medium such as water or an inorganic salt, and a reflector to reflect sunlight to collect it in the heat collection tube. In such a solar heat-collecting system, sunlight is reflected by the reflector and collected in the heat collection tube, and the heat medium in the heat collection tube is heated by the heat of such sunlight to obtain thermal energy.
Whereas, as a mirror which is commonly used indoors, a mirror is widely used which comprises a glass substrate, a reflective metal layer formed on the glass substrate and an anti-corrosive coating film layer formed on such a reflective metal layer. In such a mirror, corrosion and modification of the reflective metal layer are prevented by the glass substrate and the anti-corrosive coating film layer. As back coating materials to form the anti-corrosive coating film layer, the following compositions are, for example, disclosed.
(i) A coating composition for back coating comprising a molybdenum compound as a lead-free pigment, and a synthetic resin binder (Patent Document 1).
(ii) A coating composition for back coating comprising a metal salt such as a thiazole type metal salt, an azole type or diamine type compound, and a synthetic resin (Patent Document 2).
The back coating materials (i) and (ii) are advantageous from the environmental viewpoint, since substantially no lead-type pigment will thereby be contained in the anti-corrosive coating film layer. However, with the anti-corrosive coating film layers formed by the back coating materials (i) and (ii), no consideration is made with respect to such a severe environment of usage as a solar heat-collecting reflector is exposed outdoors for a long period of time. The following problems will result when a solar-heat collecting reflector is used outdoors for a long period of time.
(1) The anti-corrosive coating film layer is detached from the reflective metal layer by expansion or shrinkage by heat, or expansion by moisture absorption or water absorption, of the anti-corrosive coating film layer.
(2) The reflective metal layer is oxidized by moisture absorption or water absorption of the anti-corrosive coating film layer, whereby the reflectance of the reflector is deteriorated.
(3) The anti-corrosive coating film layer of a solar heat-collecting reflector is exposed in many cases, and therefore, the anti-corrosive coating film layer is likely to be deteriorated by sunlight or impingement of sand or the like.
That is, the anti-corrosive coating film layer of a solar heat-collecting reflector is required to be excellent in durability such as heat resistance, moisture resistance, water resistance, etc. to solve the problems (1) and (2), and excellent in weather resistance, scratch resistance and impact resistance to solve the problem (3).
Patent Document 2 discloses that a fluororesin is used as a synthetic resin binder to provide the durability required for the anti-corrosive coating film layer. However, simply by using a fluororesin, it is difficult to sufficiently increase the durability, weather resistance, scratch resistance and impact resistance of the anti-corrosive coating film layer, and it is difficult to solve the problems (1) to (3).